Effective high power microwave amplifier design and system modeling require accurate nonlinear models, which, in turn, require measurement of at least 3 harmonics with NIST traceable phase reference between them. Exceeding a decade of measurement bandwidth at microwave frequencies can be problematic, and a fraction of a dB of calibration error can lead to 10's of percentage points of “disappointment” in performance for the physical prototype[1]-[2]. Issues related to calibration and accurate wideband measurements for building an S Function model are presented. Pre-measurements required for efficient model extraction can include real time loadpull and “design” of the power amplifier.